git: collapse autogenerated HTML docs in GitLab

GitLab 16.10 introduced a feature to [automatically collapse
autogenerated files][1] (e. g. lock files) in merge requests.
The benefit is a more clean merge request experience, as well reducing
load on the GitLab instance.

This is also useful to enable for other kinds of autogenerated files.

In usp-js, the `public` directory is autogenerated, and is quite noisy
when viewing commits/merge requests.

Autocollapse all files in the `public` directory.

[1]: https://docs.gitlab.com/ee/user/project/merge_requests/changes.html#collapse-generated-files